This invention relates to a method and apparatus for achieving transmit diversity in telecommunication systems and, more particularly, to a method and apparatus for non-zero complex weighting and space-time coding signals for transmission on multiple antennas.
As wireless communication systems evolve, wireless system design has become increasingly demanding in relation to equipment and performance requirements. Future wireless systems, which will be third and fourth generation systems compared to the first generation analog and second generation digital systems currently in use, will be required to provide high quality high transmission rate data services in addition to high quality voice services. Concurrent with the system service performance requirements there will be equipment design constraints, which will strongly impact the design of mobile terminals. The third and fourth generation wireless mobile terminals will be required to be smaller, lighter, more power-efficient units that are also capable of providing the sophisticated voice and data services required of these future wireless systems.
Time-varying multi-path fading is an effect in wireless systems whereby a transmitted signal propagates along multiple paths to a receiver causing fading of the received signal due to the constructive and destructive summing of the signals at the receiver. Several methods are known for overcoming the effects of multi-path fading, such as time interleaving with error correction coding, implementing frequency diversity by utilizing spread spectrum techniques, or transmitter power control techniques. Each of these techniques, however, has drawbacks in regard to use for third and fourth generation wireless systems. Time interleaving may introduce unnecessary delay, spread spectrum techniques may require large bandwidth allocation to overcome a large coherence bandwidth, and power control techniques may require higher transmitter power than is desirable for sophisticated receiver-to-transmitter feedback techniques that increase mobile terminal complexity. All of these drawbacks have negative impact on achieving the desired characteristics for third and fourth generation mobile terminals.
Antenna diversity is another technique for overcoming the effects of multi-path fading in wireless systems. In diversity reception, two or more physically separated antennas are used to receive a transmitted signal, which is then processed by combining and switching to generate a received signal. A drawback of diversity reception is that the physical separation required between antennas may make diversity reception impractical for use on the forward link in the new wireless systems where small mobile terminal size is desired. A second technique for implementing antenna diversity is transmit diversity. In transmit diversity a signal is transmitted from two or more antennas and then processed at the receiver by using e.g. maximum likelihood sequence estimator (MLSE), minimum mean square error (MMSE) receivers, Maximum-a Posteriori receivers, or their approximations. Transmit diversity has more practical application to the forward link in wireless systems in that it is easier to implement multiple antennas in the base station than in the mobile terminal.
Transmit diversity for the case of two antennas is well studied. Alamouti has proposed a method of transmit diversity for two antennas that offers second order diversity for complex valued signals. S. Alamouti, xe2x80x9cA Simple Transmit Diversity Technique for Wireless Communications,xe2x80x9d IEEE Journal on Selected Areas of Communications, pp. 1451-1458, October 1998. The Alamouti method involves simultaneously transmitting two signals from two antennas during a symbol period. During one symbol period, the signal transmitted from a first antenna is denoted by S0 and the signal transmitted from the second antenna is denoted by S1. During the next symbol period, the signal xe2x88x92S1* is transmitted from the first antenna and the signal S0* is transmitted from the second antenna, where * is the complex conjugate operator. A similar diversity transmission system may also be realized in code domain. As an example, two copies of the same symbol can be transmitted in parallel using two orthogonal Walsh codes. Similar techniques can be also used to construct a space-frequency coding method.
Extension of the Alamouti method to more than two antennas is not straightforward. Tarokh et al. have proposed a method using rate=xc2xd, and xc2xe SpaceTime Block codes for transmitting on three and four antennas using complex signal constellations. V. Tarokh, H. Jafarkhani, and A. Calderbank, xe2x80x9cSpace-Time Block Codes from Orthogonal Designs,xe2x80x9d IEEE Transactions on Information Theory, pp. 1456-1467, July 1999. This method has a disadvantage in a loss in transmission rate and the fact that the multi-level nature of the ST coded symbols increases the peak-to-average ratio requirement of the transmitted signal and imposes stringent requirements on the linear power amplifier design. Additional techniques that mitigate these problems are proposed in O. Tirkkonen and A. Hottinen, xe2x80x9cComplex space-time block codes for four Tx antennas,xe2x80x9d Proc. Globecom 2000, November 2000, San Francisco, USA. Other methods proposed include a rate=1, orthogonal transmit diversity (OTD)+space-time transmit diversity scheme (STTD) four antenna method. L. Jalloul, K. Rohani, K. Kuchi, and J. Chen, xe2x80x9cPerformance Analysis of CDMA Transmit Diversity Methods,xe2x80x9d Proceedings of IEEE Vehicular Technology Conference, Fall 1999, and M. Harrison, K. Kuchi, xe2x80x9cOpen and Closed Loop Transmit Diversity at High Data Rates on 2 and 4 Elements,xe2x80x9d Motorola Contribution to 3GPP-C30-19990817-017. This method requires an outer code and offers second order diversity due to the STTD block (Alamouti block) and a second order interleaving gain from use of the OTD block. The performance of this method depends on the strength of the outer code. Since this method requires an outer code, it is not applicable to uncoded systems. For the case of rate=⅓ convolutional code, the performance of the OTD +STTD method and the Tarokh rate=xc2xe method ST block code methods are about the same. Another rate 1 method is proposed in O. Tirkkonen, A. Boariu, and A. Hottinen, xe2x80x9cMinimal non-orthogonality rate 1 space-time block code for 3+Tx antennas,xe2x80x9d in Proc. ISSSTA 2000, September 2000. The method proposed in this publication attains high performance but requires a complex receiver.
It would be advantageous, therefore, to have a method and apparatus that provided the advantage of transmit diversity on greater than two antennas while at the same time not greatly increasing the complexity of system design.
The present invention presents a method and apparatus for non-zero complex weighting and space-time coding signals for transmission on multiple antennas. The method and apparatus provides expansion of an Nxc3x97Nxe2x80x2 space-time block code, where N is the number of transmit paths and Nxe2x80x2 is the number of output symbols per transmit path, to a Mxc3x97Mxe2x80x2 space-time block code, where M greater than N, generated by using repetition and non-zero complex weighting of the symbols within the Nxc3x97Nxe2x80x2 space time block code, to allow transmission of the space time block code on a number M of diversity transmit paths. The diversity transmit paths may comprise separate antennas or beams. The temporal length of the larger code Mxe2x80x2, may equal the temporal length of the original code, Nxe2x80x2. In the method and apparatus, a transform is performed on an input symbol stream, to generate a transform result comprising a space-time block code. The N output streams of the space-time block code, each consisting of Nxe2x80x2 output symbols, are then repeated and at least one of the repeated streams non-zero complex weighted over time to generate M streams of Nxe2x80x2 output symbols for transmission on M diversity transmit paths. The non-zero complex weighting may include phase shifting.
In an embodiment, N is at least 2 and M is at least 3. At least two of the N streams of Nxe2x80x2 output symbols, corresponding to the original N streams of Nxe2x80x2 output symbols, are then each transmitted on a first at least one antenna and at least one of the Mxe2x88x92N non-zero complex weighted streams of Nxe2x80x2 symbols are transmitted on one of a second at least one antenna. The first at least one antenna and second at least one antenna may comprise of any one of the M antennas.
In another embodiment, the method and apparatus may be implemented in a transmitter having common or dedicated pilot channels that enable efficient channel estimation of the coefficients that are required to decode the space-time code. In this embodiment the common and dedicated pilot channels may be implemented alone or both together in the transmitter. In one alternative of this embodiment, training symbols are transmitted on N transmit diversity paths, making it possible to estimate the N independent diversity transmit paths. For this, a dedicated pilot channel code sequence may be multiplexed into each of the N streams of Nxe2x80x2 output symbols of the original space-time block code, to generate N streams of Nxe2x80x2 output symbols and pilot channel sequence. Repetition and non-zero complex weighting may then be applied to generate M phase shifted streams of Nxe2x80x2 symbols and pilot channel sequence. At least two of the N original streams of Nxe2x80x2 output symbols and pilot channel sequence are then transmitted on one of the first at least one antenna and at least one of the Mxe2x88x92N complex weighted streams of Nxe2x80x2 output symbols and pilot channel sequence are transmitted on one of the second at least one antenna. Another way of enabling estimation of N channels is to transmit common pilot channels so that N common pilot channel are transmitted on each of the first at least one antenna, and Mxe2x88x92N complex weighted copies of some of the N common pilot channels are transmitted on each of the second at least one antenna. The complex weighting factors used for the common channels on each of the second at least one antenna are the same as the ones used to construct the Mxe2x88x92N additional complex weighted streams of Nxe2x80x2 output symbols from the original N streams of Nxe2x80x2 output symbols. In these embodiments, the receiver may or may not know the method used to expand the Nxc3x97Nxe2x80x2 space-time block code to an Mxc3x97Nxe2x80x2 space-time block code, and the temporal weighting sequences employed.
In other embodiments, where N is at least 2 and M may be at least 3, the pilot channels may be arranged to enable estimation of at least N+1 diversity transmit paths. At least one of the N streams of Nxe2x80x2 output symbols, corresponding to the original N streams of Nxe2x80x2 output symbols, are then each transmitted on a first at least one antenna and at least one of the Mxe2x88x92N complex weighted streams of Nxe2x80x2 symbols are each transmitted on one of a second at least one antenna. Different common pilot channels are transmitted on each of the first at least one antenna and on at least one of the second at least one antenna. In these embodiments, the receiver needs at least partial knowledge of the method used to expand the Nxc3x97Nxe2x80x2 space-time block code to an Mxc3x97Nxe2x80x2 space-time block code, and the temporal weighting sequences employed.
Complex weighting in the various embodiments may be applied by applying a periodic or random complex weighting pattern to each of the symbol streams that are complex weighted. The relationship between the complex weights of the symbol streams transmitted on the various antennas may also be predefined.